1 VEGETATION CORRELATES OF GIBBON DENSITY IN THE SEBANGAU NATIONAL PARK, CENTRAL KALIMANTAN, INDONESIA Marie Hamard Oxford Brookes University September 2008 WORD COUNT: 9,022 words A dissertation submitted in partial fulfillment of the requirements of Oxford Brookes University for the degree of Master of Science
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1
VEGETATION CORRELATES OF GIBBON DENSITY IN THE
SEBANGAU NATIONAL PARK, CENTRAL KALIMANTAN,
INDONESIA
Marie Hamard
Oxford Brookes University
September 2008
WORD COUNT: 9,022 words
A dissertation submitted in partial fulfillment of the requirements
of Oxford Brookes University for the degree of
Master of Science
2
Abstract
Understanding the links between animal populations and the characteristics of their
habitat is essential for their conservation. Following recent re-assessments of the conservation
status of the Bornean agile gibbon (Hylobates albibarbis), the need to conserve the peat-
swamp forests of Kalimantan, one of its main habitats, is more pressing than ever. Little
information is available on gibbons in peat-swamp forest. The aim of this study was to
investigate the relationship between vegetation characteristics and gibbon density in a newly
protected, secondary peat-swamp forest in the Sebangau National Park. The study was
conducted from 1st May to 28
th July 2008, using auditory sampling methods and speed
plotting. Gibbon densities and vegetation characteristics were recorded at 13 sites within the
Natural Laboratory for the Study of Peat-swamp Forest. Gibbon densities were calculated
after the number of groups in each area was determined by plotting all recorded calls on a
map. Gibbon densities ranged from 1.39 to 3.92 groups/km², and were found to be correlated
with canopy cover at 20m, median tree height, density and biomass of large trees and food
availability. These results highlight the importance of large, tall trees and good canopy cover
for the gibbon population in the Sebangau forest; they indicate that conservation efforts
should be directed to the preservation of those large trees and the prevention of selective
logging, which targets them. It is hoped that the results of this survey will contribute to the
preservation of both Indonesia’s peat-swamp forests and the gibbons that inhabit them.
3
Acknowledgements
- I would like to express my gratitude to Hendri Setia Sebangau, a dedicated field assistant
without whom this research wouldn’t have been possible, and a true asset in the treacherous
peat-swamp forest!
- Thanks to my supervisor Dr Vincent Nijman for his advice and for the cheering up in times
of dissertation-related distress!
- Many thanks to the researchers and staff at Setia Alam Research station: Grace Blackham,
Benjamin Buckley, Laura Graham, Andrea Hoëing, Simon Husson, Dave Smith, Lindy
Thompson, Adul, Ambut, Ari, Dewa, Santiano, Twenti, Yudhi, for their support, their advice
and their help during data collection.
- Special thanks to the volunteers of the OUTrop volunteer program for their enthusiastic help
during data collection: Alice, Anna, Dominic, Katy, Leanne, Maria, Pau, Roz, Sarah, Simon
and Zaida.
- Thank you to my Indonesian sponsor CIMTROP, especially Dr Suwido H. Limin, for
providing logistical support and visa letters.
- This research would not have been possible without the financial support received from the
Rufford Small Grants Foundation and from the People’s Trust for Endangered Species.
- Personal thanks to Dr Susan Cheyne for providing advice and support, both technical and
emotional, before the survey, in the field and during the writing of this dissertation.
- Finally, a thousand thanks to my friends from the 2008 MSc cohort, and particularly to
Lizzie, for being there for me, for a fantastic year, and for their general loveliness.
4
TABLE OF CONTENTS
TITLE PAGE i
ABSTRACT ii
ACKNOWLEDGEMENTS iii
TABLE OF CONTENTS iv
LIST OF TABLES vi
LIST OF FIGURES vii
LIST OF ABBREVIATIONS viii
CHAPTER I: INTRODUCTION 1
I.1 Study species 1
I.2 Peat-swamp forests and study site 3
I.3 Research questions and hypotheses 5
CHAPTER II: RESEARCH PAPER 7
Abstract 8
Introduction 9
Methods 10
- Study site 10
- Estimation of gibbon density 12
- Measurement of vegetation characteristics 15
- Statistics 16
Results 17
- Calling probabilities and calculations of gibbon density 17
5
- Vegetation characteristics and determination of forest types 18
- Relationship between vegetation characteristics and gibbon density 20
Discussion 22
- Gibbon density estimates 22
- Habitat characteristics and vegetation correlates of gibbon density 25
- Implications for conservation 26
- Conclusions and recommendations 27
Acknowledgements 28
References 29
CHAPTER III: GENERAL DISCUSSION 37
- Interdependence of primates and tropical forests 37
- Recommendations for future research 39
CHAPTER IV: CONCLUSIONS AND PROPOSED CONSERVATION MEASURES 41
REFERENCES 43
APPENDICES: 50
APPENDIX I: Gibbon triangulation data sheet 50
APPENDIX II: Vegetation survey data sheet 51
APPENDIX III: List of tree species identified during the survey 52
APPENDIX IV: The training video project 55
6
LIST OF TABLES
Table 1: Hypothesised relationships between vegetation variables and gibbon densities.
Table 2: Estimates of gibbon density at each survey site and parameters used to calculate
them.
Table 3: Tree species richness and diversity at each survey site.
Table 4: Average vegetation characteristics in each forest type.
Table 5: Average gibbon density in each forest type.
Table 6: Correlation coefficients between vegetation characteristics and gibbon density.
Table 7: Comparison between Bornean agile gibbon density estimates in Kalimantan.
7
LIST OF FIGURES
Figure 1: Location of the study area in Central Kalimantan.
Figure 2: Detailed map of the study area showing the thirteen survey sites.
Figure 3: Visualisation of the variation in vegetation characteristics between forest types.
Figure 4: Correlation between the principal component analysis factor summarising
vegetation characteristics and gibbon density.
Figure 5: Relationship between gibbon density and calling probability.
8
LIST OF ABBREVIATIONS
CIMTROP Centre for the International Management of Tropical Peatlands
CITES Convention on the International Trade in Endangered Species
DBH Diameter at Breast Height
IUCN International Union for the Conservation of Nature
LAHG Laboratorium Alam Hutan Gambut (natural laboratory for the study of peat-
swamp forest)
LPF Low Pole Forest
MSF Mixed-Swamp Forest
OUTrop Orang-Utan Tropical peatland and conservation project
PCA Principal Component Analysis
PSF Peat-Swamp Forest
SNP Sebangau National Park
TIF Tall Interior Forest
UNPAR University of Palangka Raya
9
Chapter I: INTRODUCTION
Surveying primate populations is a vital conservation tool, as it gives a better
understanding of both remaining population numbers and, when associated with vegetation
studies, of habitat requirements for the animals and conservation priorities (Brockelman and
Ali, 1987; Sutherland, 2002; Ganzhorn, 2003). This study, complementing ongoing work in
the area, investigated links between the density of Bornean agile gibbons (Hylobates
albibarbis) and vegetation characteristics in a secondary peat-swamp forest.
I.1- Study species
Gibbons are small arboreal apes inhabiting the rainforests of South-East Asia,
Northwest India and Bangladesh (Reichard and Sommer, 1997). The taxonomy of gibbons is
under dispute, as the status of several taxa as species or subspecies is uncertain. Within the
family Hylobatideae, there are four genera of gibbons: Bunopithecus (hoolock gibbon),
Hylobates, Nomascus (crested gibbons) and Symphalangus (siamangs), and at least twelve
species (Brandon-Jones et al., 2004). Apart from the sympatric H. agilis/H. lar and siamangs
in Sumatra and peninsular Malaysia, gibbons are allopatric (Leighton, 1987; Marsh, 1987;
Reichard, 2000). Some hybrids have been found within the genus Hylobates, including
populations in Borneo (H. albibarbis and H. muelleri: Mather, 1992), in Thailand (H. lar and
H. pileatus, Brockelman and Gittins, 1984) and in peninsular Malaysia (H. lar and H. agilis,
Brockelman and Gittins, 1984). The Bornean agile or southern gibbon (Hylobates albibarbis)
occurs in southern Borneo, between the Kapuas and Barito rivers (Brandon-Jones et al.,
2004). Its taxonomic status is unclear, but recent molecular evidence identifies it as a separate
species, rather than a sub-species of Hylobates agilis (Groves, 2001; Brandon-Jones et al.,
2004, Geissmann, 2007).
10
Although their diet also includes young leaves and flower buds, gibbons are mostly
frugivorous (Gittins and Raemaekers, 1980; Gittins, 1982; Gittins, 1983; McConkey et al.,
2002; Cheyne 2008). Thus they appear to play a primary role in forest regeneration as high
quality seed dispersers (Gittins, 1982; McConkey et al., 2000; O’Brien et al., 2003). Gibbons
live in small family groups of two to six individuals (Gittins and Raemaekers 1980; Leighton,
1987) with an average group size of four (Gittins and Raemaekers, 1980). They are socially
monogamous, with males and females forming stable pairs (Gittins and Raemaekers, 1980;
Gittins, 1982; Leighton, 1987; Mitani, 1987; Mitani, 1990), but several long-term studies
have reported extra-pair copulations and reproductive patterns may differ from the social
system (Palombit, 1994; Reichard, 1995; Sommer and Reichard, 2000). All gibbon species
exhibit territorial behaviour, with mated pairs defending exclusive territories (Gittins and
Raemaekers, 1980; Reichard and Sommer, 1997). Home ranges may however overlap,
especially in areas where gibbon densities are high (Gittins and Raemaekers, 1980; Mitani,
1990; Reichard and Sommer, 1997). Most gibbon pairs use a sequence of calls called a duet to
defend their territory and strengthen their pair bond (Mitani, 1987; Cowlishaw, 1992;
Geissmann and Orgeldinger, 2000): in the morning the resident pair of each territory utters a
series of calls, the more characteristic and most easily recognisable one being the female’s
great call (Brockelman and Srikosamatara, 1993). It must be noted that the Kloss gibbon
(Hylobates klossii) and the Moloch or Javan gibbon (Hylobates moloch) do not duet, though
both sexes do still sing (Bartlett, 2007). This great call, which can be heard up to one
kilometre away in flat, dense rainforest (Brockelman and Ali, 1987), can be used for auditory
sampling method in surveys (Brockelman and Ali, 1987; Brockelman and Srikosamatara,
1993; Nijman and Menken, 2005; Buckley et al., 2006; Cheyne et al., 2008).
All Indonesian gibbons are faced with threats to their survival, both because of habitat
loss through logging, encroachment and forest fires, and because of hunting for food or
11
capture for the pet trade, and are placed on the CITES Appendix 1. Hylobates albibarbis was
listed, as a subspecies of H.albibarbis, as Lower Risk/near threatened as a sub-species of H.
agilis in the IUCN Red list in the last assessment (IUCN Red List 2006). Following new
genetic research and on the recommendation of the Asian Primate Red List Workshop, held in
2006 in Phnom Penh, Cambodia, the species’ status has been recognised as Hylobates
albibarbis and classified as Endangered, partly because of the rapid rate of destruction of
peat-swamp forests, which constitute a large part of its range (Geissman, 2007). Although
much is known about the behaviour and ecology of gibbons, limited data are available on the
population status of H. albibarbis, and the Asian Primate Red List Workshop concluded that
more recent population estimates and rigorous monitoring of those populations are needed for
the species’ conservation (Geissman, 2007). Although gibbons have been the focus of many
Mitani, 1990; Bartlett, 2007), there is little information on their habitat requirements, in
particular in peat-swamp forests (Buckley et al., 2007). No clear explanation for the important
variation in gibbon densities between field sites has been established (Leighton, 1987); since
vegetation correlates of primate densities have been found for other species (e.g. Ross and
Srivastava, 1994; Muoria et al., 2003; Wieczkowski, 2004; Nekaris et al., 2005; Rovero and
Struhsaker, 2007), it is possible that they exist for gibbon populations as well.
I.2- Peat-swamp forests and study site
As acknowledged in recent workshops on the conservation status of gibbons in
Indonesia (Geissmann, 2007, Campbell et al., 2008), one of the major habitats of the Bornean
agile gibbon is peat-swamp forest. Peat-swamp forests (PSF) occur in the Indo-Malayan
region, principally in Kalimantan and Sumatra (Page et al., 1997; Page, 2002). Kalimantan
has extensive peatlands, covering about 6 million hectares of its lowlands (Rieley et al.,
12
1997), a very small proportion of which is protected within national parks (Morrogh-Bernard
et al., 2003). Peat-swamp forests are seasonally flooded, waterlogged lowland forests.
Because they were thought to harbour little biodiversity, and because they contain tree species
of commercial interest (Rieley et al., 1997; Page, 2002; Morrogh-Bernard et al., 2003), PSF
have received little conservation attention and have been extensively cleared and/or converted
to cultivated land. Logging activities, in addition to the removal of large trees, are detrimental
to the ecosystem as the canals dug within the forest to carry felled trunks to adjacent rivers
drain the peat, making the soil dry and prone to wildfires (Morrogh-Bernard et al., 2003;
Cheyne, 2007). However, more recent studies have highlighted the importance of PSF for
conservation. Despite their waterlogged, highly acidic, nutrient-poor soil, PSF have a tree
species diversity comparable to other forests on mineral soils, and feature a number of
commercially valuable trees (Rieley et al., 1997; Felton et al., 2003). Reports on the fauna in
PSF are scarce but surveys have recorded 57 species of mammals, 237 species of birds, 55
species of fish, as well as reptiles and amphibians (Page, 2002). In Central Kalimantan,
between the Kapuas and Barito rivers, lies the protected peat-swamp forest of the Sebangau
National Park (SNP). This area covers 5,300km² and was gazetted as a national park in
November 2004, after having been allocated to logging companies for timber extraction for
thirty years. Previous work in the area has focused on ecological and hydrological studies as
well as forest regeneration monitoring (Page et al., 1999) and has shown that the Sebangau
catchment harbours the largest remaining wild population of Bornean orang-utans (Pongo
pygmaeus) (Morrogh-Bernard et al., 2003) and an extensive wild population of Bornean agile
gibbons (Hylobates albibarbis) (Buckley et al., 2006; Cheyne et al., 2008). Ongoing work is
being carried out on behavioural aspects and feeding ecology of gibbons and orang-utans in
PSF, aiming to fill gaps in scientific knowledge about apes in this unique ecosystem (Page,
2002). Survey work is also ongoing on populations of nocturnal primates (Bornean slow loris
13
Nycticebus coucang menagensis and western tarsier Tarsius bancanus borneanus)(Blackham,
2005; Nekaris et al., 2008), flying foxes (Pteropus vampyrus natunae) (Struebig et al., 2007)
and wild felids (Cheyne, personal communication).
I.3- Research questions and hypotheses
Previous survey efforts on gibbons in the area primarily focused on a 4km² site which
has been intensely studied (Buckley et al., 2006; Cheyne et al., 2008). However there was a
lack of data on gibbon densities in other parts of the Sebangau peat-swamp forest. The first
aim of this study was to extend survey efforts to a larger set of sites, encompassing several
forest types within the Sebangau PSF, in order to obtain more reliable population estimates.
Secondly, measurements of vegetation characteristics at each survey site were taken to help
identify possible differences between forest types. Thirdly, as vegetation correlates of primate
densities have been found in other species (Ross and Srivastava, 1994; Wieczkowski, 2004;
Rovero et al., 2007), the final aim of this study was to investigate whether such correlates
could be found between gibbon densities and vegetation characteristics in the Sebangau PSF
and to identify the most influential of those correlates.
The hypotheses which were formulated prior to the study were:
- that gibbon density estimates within the main 4km² study area would be comparable
between this study and the two previous surveys;
- that gibbon density would differ between study sites and between forest types;
- that vegetation characteristics would show some variation across sites and between forest
types;
- that gibbon density could be correlated to vegetation characteristics, as shown in Table 1.
14
Table 1: Hypothesised relationships between vegetation variables and gibbon densities.
Variable name Expected relationship to gibbon density Justification Reference
Mean canopy cover at 20m Gibbon density higher where canopy cover is higher
Gibbon are exclusively arboreal and use continuous canopy for their locomotion
Johns, 1986; Bartlett, 2007
Median tree height Gibbon density higher where trees are higher Gibbon prefer to use higher canopy levels. High trees are used for singing and sleeping.
MacKinnon and MacKinnon, 1980; Whitten, 1982; Gittins, 1983; Mather, 1992
Mean DBH Gibbon density higher where trees are larger
Density of large trees Gibbon density higher where large trees are numerous
Large trees provide more canopy cover and represent more abundant food sources.
Cowlishaw and Dunbar, 2000
Total cross-sectional area of all trees
Gibbon density higher where total biomass of trees is higher
Total cross-sectional area of large trees
Gibbon density higher where biomass of large trees is higher
Gibbons are exclusively arboreal. A high biomass of trees means a larger number of trees and/or the presence of large trees.
Bartlett, 2007
Total cross-sectional area of gibbon food trees
Gibbon density higher where food availability is higher
Gibbon density has been found to be correlated to food availability at other field sites.
Mather, 1992; Chivers, 2001; Cheyne et al., 2006
Total cross-sectional area of top 20 gibbon food trees
Gibbon density higher where biomass of preferred food trees is higher
There are indications that the availability of a small number of tree species might be used as a correlate of gibbon density.
Mather, 1992
15
CHAPTER II: RESEARCH ARTICLE 1
2
Vegetation correlates of gibbon density in the peat-swamp forest of 3
Sebangau national park, Central Kalimantan, Indonesia. 4
5
Marie Hamard1, Susan M. Cheyne
2, Vincent Nijman
1 6
7
1 School of Social Science and Law, Oxford Brookes University 8
2 Wildlife Conservation Research Unit, University of Oxford 9
10
Short title: Vegetation correlates of gibbon density 11
12
Correspondence should be addressed to Marie Hamard at Department of 13
Anthropology, School of Social Science and Law, Oxford Brookes University, 14
Headington Campus, Gypsy Lane, 15
Oxford OX3 0BP, UK 16
17
For submission to: American Journal of Primatology18
16
ABSTRACT 19
Understanding the complex relationship between primates and their habitats is 20
essential for effective conservation plans. Peat-swamp forest has recently been recognized as 21
a major habitat for the Bornean agile gibbon (Hylobates albibarbis), but information is scarce 22
on the factors that link gibbon density to characteristics of this unique ecosystem. The aims of 23
this study were firstly to estimate gibbon density in different forest subtypes in a newly 24
protected, secondary peat-swamp forest in the Sebangau National Park, Indonesia, and 25
secondly to identify vegetation correlates of gibbon density. Data collection was conducted 26
for three months at 13 survey sites, using auditory sampling methods and speed plotting. 27
Gibbon densities were found to vary from 1.39 to 3.92 groups/km² between survey sites in 28
different forest subtypes. Canopy cover, tree height, density of large trees and food 29
availability correlated with gibbon density, identifying the preservation of tall trees and good 30
canopy cover as a conservation priority for the gibbon population in the Sebangau forest. This 31
survey indicates that selective logging, which specifically targets large trees and disrupts 32
canopy cover, is likely to have adverse effects on gibbon populations in peat-swamp forests, 33
and calls for greater protection of these little studied ecosystems. 34
Peat-swamp forests have received little conservation interest in the past, as their 37
richness in biodiversity is generally lower than other types of lowland rainforests (Page et al 38
1997). However, recent research has indicated that they harbor some of the largest remaining 39
populations of Bornean orangutans (Pongo pygmaeus pygmaeus) (Felton et al., 2003; 40
Morrogh-Bernard et al. 2003) and Bornean agile gibbons (Hylobates albibarbis) (Buckley et 41
al., 2006; Geissmann, 2007; Cheyne et al., 2008), two species of endangered primates 42
endemic to Borneo. 43
Large extents of peat-swamp forest exist in Kalimantan, but the level of protection of 44
these ecosystems is low. Because these forests contain commercially valuable timber at high 45
densities, they are often commercially logged, either legally or illegally (MacKinnon et al., 46
1997a). In addition to the removal of large trees, logging also has severe consequences for the 47
ecosystem’s balance, as canals are created to carry fallen trunks to the river, and lead to the 48
drainage of water from the peat, which is then very vulnerable to forest fires (Morrogh-49
Bernard et al., 2003). Finally, large extents of peat-swamp forests are cleared for agriculture 50
(Rieley et al., 1997). To combat these threats, considerable research efforts were made in the 51
peat-swamp forest of the Sebangau catchment, Central Kalimantan, to highlight its 52
importance both as a biodiversity-rich ecosystem (Page et al., 1997; Morrogh-Bernard et al., 53
2003) and as an important purveyor of ecological services such as carbon and water storage, 54
and climate regulation (Rieley et al., 1997; MacKinnon et al., 1997b; Morrogh-Bernard et al., 55
2003; Cheyne, 2007). These efforts led to the creation of the Sebangau National Park, which 56
was gazetted in 2004, and the cessation of the legal logging activities which had been taking 57
place for thirty years. However, protection of the peat-swamp forest is not enforced in many 58
parts of the park where anthropogenic disturbance continues. Density and population 59
estimates yielded by previous studies in the area demonstrate its importance for gibbon 60
18
conservation, as the area was shown to host at least 25,000 wild gibbons (Buckley et al., 61
2006; Cheyne et al., 2008). 62
The ability to assess accurately the population status of threatened species is of prime 63
importance to conservation strategies (Sutherland, 2000). Moreover, as the survival of 64
primates is connected to and dependent upon the protection of their rainforest habitats (Lucas 65
and Corlett, 1998; Chapman et al., 2006; Link and Di Fiore, 2006), understanding links 66
between primate abundance and the characteristics of their habitat is essential to effectively 67
conserve the resident primates. 68
The aims of this study were: first, to survey the population of Bornean agile gibbons 69
around the Setia Alam research station, in the north of the Sebangau catchment. Two density 70
estimates have been published so far for this gibbon population, with previous survey efforts 71
concentrating mainly on a 4km² study site (Buckley et al., 2006; Cheyne et al., 2008). This 72
study included a repeat of these survey efforts in order to monitor gibbon numbers in the area. 73
Secondly, this study was designed to identify relationships between gibbon density and 74
habitat quality, by extending survey efforts to several sites in the forest, which exhibit 75
different vegetation characteristics, in order to identify important habitat requirements for 76
gibbons in the unique peat-swamp forest ecosystem. 77
78
METHODS 79
Study site 80
The Sabangau National Park, designated in November 2004, covers 568 000 hectares of 81
peat-swamp forest between the Sabangau and Katingan Rivers in Central Kalimantan. The 82
study area is located within the Natural Laboratory for the Study of Peat-swamp Forest 83
(Laboratorium Alam Hutan Gambut, LAHG), a 500km² area situated in the north of the 84
19
national park, 20 km south-west of Palangkaraya, the provincial capital of Central 85
Kalimantan (Fig. 1). This area is operated by the Centre for International Cooperation in 86
Sustainable Management of Tropical Peatland (CIMTROP). Within this area, the main 87
research station, Setia Alam Field Station, is located 2° 20’ 42’’ south and 114° 2’ 11’’ 88
east. At the time of the field surveys the area was protected and timber extraction had 89
stopped at the survey locations. Small-scale harvesting of non-timber forest products (bark, 90
rattan, orchids and latex) was still taking place. Previous studies have identified three main 91
peat-swamp forest sub-types within the study area based on tree species composition and 92
forest structure: a mixed swamp forest (MSF) at the periphery, which transitions into low 93
pole forest (LPF) about 6km inside the forest, turning into tall interior forest at the centre 94
of the peat dome, about 12km inside the forest (Shepherd et al., 1997; Page et al., 1999; 95
Morrogh-Bernard et al., 2003). The forest is secondary closed-canopy evergreen forest, 96
which is flooded for most of the year, and is home to a high diversity of mammals 97
(Morrogh-Bernard et al., 2003) and birds (Page et al., 1997). 98
99
100
101
102
103
104
105
106
107
Figure 1: Location of the study area in Central Kalimantan, in the northern part of the Sebangau catchment, between the Kahayan river to the east and the Katingan river to the west LAHG: Natural Laboratory for the Study of Peat-
Swamp Forest.
20
The study area is on the site of a former logging concession, and the existing access route into 108
the forest, formerly a railway, is still used by the researchers. An additional network of 109
marked transects has been cut for behavioral studies and was also used for this study. 110
Estimation of gibbon density 111
Gibbon density was estimated using fixed-point counts, as described by Brockelman 112
and Ali (1987), at fourteen distinct survey sites within the research area (Figure 2). This 113
method has been recommended for the survey of gibbons for the following reasons: first, 114
because the gibbons’ inconspicuous behavior and preference for high canopy makes the use of 115
line transects for surveying unsuccessful (Brockelman and Ali, 1987; Brockelman and 116
Srikosamatara, 1993; Nijman and Menken, 2005). Secondly, because the territorial behavior 117
of gibbons allows efficient mapping of triangulated points (Sutherland, 2000). The animals’ 118
loud calls, audible from a considerable distance, allow their detection from greater distances 119
than by using sightings (Davies, 2002). Finally, fixed-point counts allow quick, time-efficient 120
surveys, with more reliable results than a line transect survey conducted within the same time 121
frame (Nijman and Menken, 2005). This method has proved efficient in several primate 122
surveys (e.g. Brockelman and Srikosamatara, 1993; Gursky, 1998; Estrada et al., 2002; 123
Estrada et al., 2004; Nijman, 2004). The method has also been successfully used in two 124
previous survey at the study site (Buckley et al., 2006; Cheyne et al., 2008), allowing the 125
comparison of their results to those yielded by the present survey. 126
21
127
128
Data collection took place between May 7th
and July 27th
, 2008 at nine sets of listening 129
posts. Additional data were obtained from previous studies for two of the sites within the 130
main grid system (Cheyne et al., 2008), and collected in the summer of 2007 for the two 131
remaining survey sites. 132
The compass bearings and estimated distances of gibbon calls were recorded from 133
three listening posts situated in a triangle formation, with a distance of 300 to 600m between 134
them, for four consecutive days at each survey site. Previous surveys have shown that the use 135
of three listening posts for auditory sampling yields the best results as it allows better 136
mapping of the gibbon groups (Nijman et al., 2005; Cheyne et al., 2008). Data collection took 137
place between 04:30h and 08:00h each morning, excluding rainy mornings and mornings for 138
which rain had stopped less than two hours before the planned start of data collection, as rain 139
T3 T4
T7 T8
T12 s
T12 n
Kora
n
0 10 km
Sebangau River Jelutong
Transect or path
River
Set of listening posts
Survey camp
Setia Alam Research station
Forest edge
Bakung River
Koran
Figure 2: Detailed map of the study area, showing the thirteen sets of listening posts used
for auditory sampling. Vegetation sampling was conducted at the same sites, along
transects. Relevant transect names are indicated by the letter T.
22
has been found to influence negatively the gibbons’ singing behavior (Brockelman and Ali, 140
1987; Brockelman and Srikosamatara, 1993). Because of this, the survey period was reduced 141
to three days at three of the survey sites, and two days at one survey site. However, correction 142
factors included in the formula to estimate density ensured the data were comparable between 143
all survey sites (Brockelman and Ali, 1987; Brockelman and Srikosamatara, 1993). All data 144
collection was conducted by experienced researchers and volunteers after a training period of 145
at least a week; whenever possible, teams of researchers rather than a single individual were 146
used to maximize the accuracy of compass bearings. After plotting all recorded singing bouts 147
on a map using Microsoft Excel, the number of groups within each surveyed location could be 148
determined by triangulation or quadrangulation. Only groups for which at least one great call, 149
indicating a family group, was heard during the survey time were included in the analysis, in 150
order to avoid counting solitary animals (Brockelman and Ali, 1987). 151
The density estimates were obtained with the following formula, developed by 152
Brockelman and Ali (1987): D = n / [p(m) x E], where n is the number of groups heard in an 153
area as determined by the mapping, p(m) is the estimated proportion of groups expected to 154
sing during a sample period of m days, and E is the effective listening area. The correction 155
factor p(m) was determined at each site with the formula p(m) = 1 – [1- p(1)]m
, with p(1) 156
being the singing probability for any given day, and m being the number of survey days. The 157
effective listening area was calculated for each site using a fixed radius of 1km around each 158
listening post, and was defined by the area in which at least two of the listening posts could 159
hear gibbons singing. Areas which were not covered in forest (outside the forest edges and in 160
burnt areas) were deducted from the effective listening area using maps. The total survey 161
effort covered 37.1 km² across the three main forest types, during 49 survey days. 162
23
Measurements of vegetation characteristics 163
Habitat characteristics were measured in plots of 10m x 10m situated along transects 164
around the listening posts, in the same time frame as the auditory sampling. Previous studies 165
investigating relationships between forest structure and primate densities have used small 166
plots (e.g. Rendigs et al., 2003; Blakham, 2005). Ten plots per site were analyzed, with the 167
exception of five sites within the grid system, for which six plots were analyzed because of 168
time constraints. In each plot the following data were recorded: 1/ canopy cover at 20m, at 169
each corner and in the middle of the plot, using visual estimation by the same observer 170
throughout the survey; 2/ diameter at breast height (DBH) of all trees exceeding 10cm DBH; 171
3/ height of all trees exceeding 10cm DBH, placing each tree into classes from 0-5m to 35m+ 172
by visual estimation by trained researchers; 4/ local name of the species of all measured trees; 173
5/ total number of trees in the plot. Additional data were obtained for two of the sites (Koran 174
and Jelutong, see Figure 2) for which 100m x 5m plots were used by another team of 175
researchers in 2007 and only DBH and tree species were recorded. DBH was then converted 176
into cross-sectional area using the formula cross-sectional area = (DBH/2)² x π and used as an 177
indicator of tree biomass. 178
All the collected data were then summarized into nine variables for each plot: 1/ mean 179
canopy cover, 2/ median tree height, 3/ mean DBH, 4/ density of all trees ≥10cm DBH, 5/ 180
density of large trees (≥ 20cm DBH), 6/ total cross-sectional area of all trees ≥10cm DBH, 7/ 181
total cross-sectional area of large trees (≥ 20cm DBH), 8/total cross-sectional area of known 182
gibbon food trees, 9/ total cross-sectional area of trees belonging to the 20 species eaten most 183
frequently by gibbons in the area. Gibbon food trees were defined as tree species whose 184
edible parts (leaves, flowers, fruits, seeds) are known to be eaten by gibbons in the area. 185
Information to obtain the two last variables were based on long-term behavioral research on 186
the gibbons’ feeding ecology in the area (Cheyne and Shinta, 2006; Cheyne, 2008). Tree 187
24
species were identified by Hendri Setia Sebangau, a local field assistant with extensive 188
knowledge of forestry in the area. 189
All vegetation characteristics were then averaged for each study site, except median tree 190
height which was directly calculated for all measured trees within a study site. Measures of 191
species diversity were then added to the analysis: species richness, defined by the number 192
of tree species identified in each study site; Shannon-Wiener’s diversity index and 193
Simpson’s diversity index, calculated as described in Ganzhorn (2003) and Douglas 194
(2006). Both Shannon-Wiener and Simpson’s indexes were calculated, as both are biased 195
towards either dominant species (Simpson’s index) or rare species (Shannon-Wiener 196
index) (Stiling, 2002). 197
Statistics 198
Vegetation characteristics between sites were compared using Kruskal-Wallis 199
ANOVA non-parametric test. Pair-wise comparisons of means for each of the variables were 200
carried out between forest types using Mann-Whitney’s U test. After testing for the normality 201
of each variable using Kolmogorov-Smirnov test, potential correlation between gibbon 202
density and each of the variables obtained from vegetation characteristics was investigated 203
using Pearson’s correlation test. Correlation was preferred to regression analysis, as no 204
relationship of cause and effect between vegetation characteristics and gibbon density was 205
assumed. A factor analysis was also performed to obtain a single component retaining most of 206
the variation contained in the vegetation data set. All tests were carried out using SPSS v.16, 207
with a significance level of p<0,05. Standard error, which is used to assess the accuracy of 208
calculated means in the population (Fowler et al., 1998), was used to measure variability in 209
the analysis, rather than standard deviation. 210
25
RESULTS 211
Calling probabilities and calculations of gibbon density 212
Based on the number of groups calling on each sampling day and the total number of 213
groups heard for each site, the probability for a group to be calling on any given day p(1) was 214
calculated. The cumulative probability of hearing all gibbon groups during a sample period of 215
m days, p(m), was deducted from p(1) as described in the methods section. Table 2 216
summarizes the parameters of calling probabilities and effective listening areas for all survey 217
sites, as well as resulting gibbon density estimates. 218
Table 2: Parameters calculated for the estimation of gibbon density at each site, and resulting density 219 estimates. Reduced sampling periods (m) were due to weather conditions. Reduced effective listening 220 areas (E) were due to forest edges or areas of forest destroyed by wildfires. 221 222
Site name and site number Number of groups heard p(1) m
(days) p(m)
E
(km²)
Gibbon density
(groups/km²)
Camp (MSF) 5 0.67 5 1.00 1.97 2.53
Middle (MSF) 8 0.53 4 0.95 3.12 2.69
Old Railway (MSF) 8 0.59 4 0.97 2.78 2.96
Hell (MSF) 7 0.67 5 1.00 2.85 2.81
Km 2 (MSF) 7 0.64 4 0.98 2.86 2.49
Km 3.5 (transition) 7 0.64 4 0.98 3.10 2.30
Km 4 (transition) 5 0.55 4 0.96 3.06 1.71
Km7 (LPF) 3 0.33 3 0.70 3.08 1.39
Km 8 (LPF) 4 0.50 2 0.75 3.13 1.70
Km 12 North (TIF) 8 0.54 3 0.90 2.26 3.92
Km 12 South (TIF) 8 0.54 3 0.90 3.04 2.91
Koran (MSF) 6 0.5 4 0.94 3.00 2.41
Jelutong (MSF) 7 0.5 4 0.94 2.86 2.61
223
The density estimates given above are in groups per square kilometer, as no determination of 224
the average group size in the area was attempted during this survey. However, previous 225
research in the main study area has established an average group size of 4.05 for gibbons in 226
the MSF (Cheyne et al., 2008). Using this group size, the density estimate yielded by this 227
study for the MSF is 10.70 ± 0.19 individuals/km². 228
26
Vegetation characteristics and determination of forest types 229
Species richness and diversity indicators for each of the survey sites are shown in 230
Table 3. Only sites for which vegetation information was obtained from 10x10m plots are 231
included in the analysis. A total of 61 species or groups of species of trees were identified 232
during this study, representing 33 families (Appendix III). 233
Table 3: Tree species richness and diversity at each study site 234 235
Site name Species richness
S
Shannon-Wiener index
H
Evenness
J
Simpson’s index
C
Camp (MSF) 24 3,04 0,96 0,03
Hell (MSF) 20 2,79 0,93 0,06
Old railway (MSF) 27 3,05 0,93 0,05
Middle (MSF) 34 3,22 0,91 0,04
km2 (MSF) 32 3,11 0,90 0,05
km3,5 (transition) 29 2,81 0,84 0,09
km4 transition) 31 3,11 0,91 0,05
km7 (LPF) 20 2,66 0,89 0,07
km8 (LPF) 22 2,60 0,84 0,09
km12N (TIF) 26 2,68 0,82 0,11
km12S (TIF) 26 2,43 0,74 0,19
236
Three main forest types can be identified in the Sebangau peat-swamp forest: a low pole 237
forest (LPF) with short, small trees, a very scarce canopy cover at 20m and few large trees (≥ 238
20cm DBH); a tall interior forest (TIF) with high, large trees, high canopy cover and high 239
gibbon food availability; and a mixed-swamp forest (MSF), situated closest to the river, with 240
a more heterogeneous vegetation. Two survey sites, situated between MSF and LPF, were 241
labeled transition forest. Overall, the vegetation in sites situated in the MSF exhibits high 242
species richness (average s = 27.4) and contains evenly distributed, relatively rare species 243
(average J = 0.92), which results in high Shannon-Wiener indexes (average H= 3.04) and low 244
Simpson’s indexes (average C = 0.05). Sites in LPF exhibit poor species richness (average s = 245
21) and low species diversity (average H = 2.63; average C = 0.08). Finally, TIF vegetation is 246
species-rich (average s = 26) but unevenly distributed (average J = 0.78), with notably 247
Palaquium leiocarpum (hangkang) trees dominating in both sites and representing 29% and 248
27
43% of the trees in sites km12N and km12S, respectively. This results in a high average 249
Simpson’s index (C = 0.15) and a low Shannon-Wiener index (H = 2.65) for TIF. 250
All vegetation variables, averaged for each forest type, are presented in Table 4. 251
Table 4: Average vegetation characteristics for the forest types of the Sebangau peat-swamp forest. All 252 values are given with standard errors. 253 254
Forest type
Mean
canopy
cover (%)
Median
tree height
(m)
Mean DBH
(cm)
Density
large trees
(trees/ha)
Biomass
all trees
(cm²)
Biomass
large trees
(cm²)
Biomass
food trees
(cm²)
Biomass
top20 food
trees (cm²)
MSF
(n=42)
40.9
±3.8 11-15
16.3
±0.5
231.2
±24.9
2546
±190
1443
±178
2018
±167
1012
±107
Transition
(n=20)
20.8
±3.9 11-15
15.5
±0.7
220.0
±49.5
3332
±569
1852
±545
2469
±404
1113
±200
LPF
(n=20)
10.0
±1.6 11-15
16.0
±0.7
150.0
±28.0
2094
±254
1067
±218
1712
±251
572
±93
TIF
(n=20)
61.8
±3.4 16-20
19.4
±0.9
385.0
±41.8
4198
±419
3104
±442
3455
±357
1037
±210
Kruskal-Wallis
χ² and p
χ² = 49.0
p=0.001
χ² = 22.5
p=0.001
χ² = 14.0
P=0.003
χ² = 18.2
p=0.001
χ² = 17.4
p=0.001
χ² = 16.4
p=0.001
χ² = 16.9
p=0.001
χ² = 8.1
p=0.04
255
Significant differences were found between forest types for all variables. Pair-wise 256
analysis revealed that MSF and transition forest had similar floristic characteristics except for 257
canopy cover, which was significantly higher in MSF (U=173.5, p=0.004). MSF also had 258
significantly higher canopy cover (U=72, p=0.01) and median tree height (U=189, p=0.01) 259
than LPF and contains more of the top 20 gibbon food trees (U=231, p=0.004). Canopy cover 260
(U=126, p=0), median tree height (U=181.5, p=0.007), density of large trees (U=187, p=0), 261
total biomass of trees (U=197, p=0.001) and large trees (U=189.5, p=0.001) were all 262
significantly higher in TIF than in MSF, as was total biomass of gibbon food trees (U=174, 263
p=0.01). The biomass of the top 20 gibbon food trees did however not differ between TIF and 264
MSF. 265
28
266
Relationship between vegetation characteristics and gibbon density 267
Average gibbon densities were calculated for each forest type identified previously, as 268
presented in table 5. 269
Table 5: Average gibbon densities for the forest types of the Sebangau peat-swamp forest. The values 270 obtained for transition forest, LPF and TIF are indicative values only, are the sample size is too small to 271 be able to calculate a standard error. 272 273
Forest type Gibbon density (groups/km²)
MSF
(n=7)
2.64 ± 0.07
Transition
(n=2)
2.00
LPF
(n=2)
1.54
TIF
(n=2)
3.42
274
All vegetation variables had a normal distribution, as did gibbon density (Z = 0.6, p = 0.864 275
for gibbon density). Gibbon density was found to be correlated to all the measured vegetation 276
variables, except the density of all trees and the biomass of the top 20 gibbon food trees 277
(Table 6). 278
Table 6: Correlation between gibbon density and habitat characteristics across study sites 279
APPENDIX II: VEGETATION CHARACTERISTICS DATA SHEET
HUTAN QUALITAS / HABITAT CHARACTERISTICS Tempat / Site: Nomor Plot /Plot number: GPS : Arah sebela plot /Directions of sides: Lokasi plot / Location of plot: